Typical photographic elements use silver halide emulsions, the silver halide having a native sensitivity to ultraviolet UV radiation ("UV"). Such UV sensitivity is usually undesirable in that it produces an image on the photographic element which is not visible to the human eye. In addition, in the case of color photographic elements, in particular, color dye images formed on the light sensitive emulsion layers by color development easily undergo fading or discoloration due to the action of UV. Also, color formers, or so-called couplers, remaining in the emulsion layers are subject to the action of UV to form undesirable color stains on the finished photographs. The fading and the discoloration of the color images are easily caused by UV of wavelengths near the visible region, namely, those of wavelengths from 300 to 400 nm. For the foregoing reasons, photographic elements typically incorporate a UV absorbing material in an uppermost layer (that is, the layer that will first receive radiation when the element is in normal use).
Many types of UV absorbing materials have been described previously, and include those described in U.S. Pat. Nos. 3,215,530, 3,707,375, 3,705,805, 3,352,681, 3,278,448, 3,253,921, and 3,738,837, 4,045,229, 4,790,959, 4,853,471, 4,865,957, and 4,752,298, and United Kingdom Patent 1,338,265. Known UV absorbing materials often have many undesirable characteristics. For example, they tend to color and form stains due to their insufficient stability to UV, heat, and humidity. Also, a high-boiling organic solvent is usually required for the emulsification of the UV absorbing agents, which softens the layer and substantially deteriorates interlayer adhesion. In order to prevent these problems, a large amount of gelatin has been used in the layer containing the UV absorbent, resulting in a layer which may be unstable. Alternatively, a separate gelatin protective layer was provided over the UV absorbent containing layer. Such approach results in an undesirable thickening of the element. Furthermore, previously known UV absorbing agents, when provided in the uppermost layer of a photographic element, often migrate and crystallize at the surface of the layer. Thus, a gel overcoat would be used to minimize this undesirable blooming phenomenon. Furthermore, the droplets of such UV absorbing materials, when prepared by the conventional emulsification method described above, usually have particle sizes greater than 200 nm thereby producing light scattering with resulting deterioration of the element's photographic properties. The toxicity of such UV absorbing agents has also become an important issue recently.
It is known that polymer latexes obtained by polymerization of UV absorbing monomers, can be utilized as UV absorbing agents which do not have many of the disadvantages described above. At least three methods of adding polymeric UV absorbing agents in the form of latex to hydrophilic colloid composition, are known. The first method comprises adding a latex prepared by emulsion polymerization directly to a gelatin-containing silver halide emulsion. Emulsion polymerization is well known in the art and is described in F. A. Bovey, Emulsion Polymerization, issued by Interscience Publishers Inc. New York, 1955. This is the most direct way of preparing a polymer latex.
The second method of forming a polymer latex is by solution polymerization of monomer mixture comprising UV absorbing monomer and hydrophobic comonomers. An organic solvent is used for dissolving the hydrophobic polymeric UV absorbing agent and the solution is dispersed in an aqueous solution of gelatin in the form of latex. Polymeric UV absorbing polymer latexes prepared by the foregoing two processes have been described in, for example, U.S. Pat. Nos. 3,761,272; 3,745,010; 4,307,184; 4,455,368; 4,464,462; 4,513,080; 4,340,664; GB 1,504,949; GB 1,504,950; British Patent 1,346,764; EP Application 0 190 003 and others. The third method of forming polymer latex is by solution polymerization of a monomeric mixture of UV absorbing monomer, a comonomer, and an ionic comonomer containing sulfonic, sulfuric, sulfinic, carboxylic or phosphoric acid, and their metal salts, such as acrylamido -2,2'-dimethyl-propane sulfonic acid, 2-sulfoethyl methacrylate, or sodium styrene sulfonate. The polymer solution obtained is then dispersed in aqueous solution and forms a latex.
Although the UV absorbing polymer latexes previously known have several advantages as described above, they also have the following problems.
1. The polymeric UV absorbing agent itself is not sufficiently stable to UV, heat, and humidity resulting in lowered UV protection for the element.
2. Some polymeric UV absorbing agents absorb light beyond 400 nm and cause yellow coloration which is particularly objectionable in unexposed areas of the photographic elements.
3. The UV absorbing monomers have low solubility and very poor polymerization ability.
4. The absorption characteristics and the extinction coefficient in the range of 300 nm to 400 nm is poor. This contributes to light induced stain and poor protection of the image dyes from fading upon exposure to sunlight.
UV absorbers for plastic materials, are well known. For example, U.S. Pat. No. 4,528,311 describes a UV absorbing polymer compositions comprising copolymers of 2-hydroxy-5-acrylyloxyalkylphenyl-2H-benzotriazoles which are used in the manufacture of intraoccular lenses and contact lenses. The monomer is incorporated into the lens forming polymer to provide UV absorbing properties. However, UV absorbers which may be effective in one environment, such as in the plastic of an intraocular lens, may not be effective in a totally different environment such as the hydrophilic colloidal gelatin layer of a photographic element.
It would therefore be desirable to provide a photographic element containing a polymeric UV absorber, which absorber is itself relatively stable in the typical photographic element environment, which is readily produced by polymerization from the monomer, and which has a high extinction coefficient in the 300 nm to 400 nm range while not significantly absorbing light beyond 400 nm, and which provides good UV protection of image dyes from exposure to UV.